The present invention relates generally to a cementitious board system and to methods for producing cementitious board.
The use of cementitious materials, such as, but not limited to, calcium sulfate material and Portland cement, is well known in the art for inclusion in various products such as wallboard, acoustical panels, and cement board. A substantially uniform thickness across the length and width of such cementitious boards or panels is desirable for their use in side-by-side arrays on walls, ceilings, or floors. Control of the thickness by means of screeding devices is limited by the flow properties of the hydraulic cement mixture. Cementitious slurries are usually thixotropic but often do not yield quickly enough to a screed laid across a fast moving conveyor belt to be spread evenly. Irregularity in the amount of slurries deposited on a fast moving conveyor belt tends to cause unevenness in the so-called “cement boards” and other building panels manufactured on high-speed production lines.
Cementitious building panels are made commonly in widths of from 24 to 54 inches. The wider the panel, the more difficult is the problem of even distribution of the slurry. The discharge of cementitious slurry onto a moving support surface directly from a continuous mixer could present a continuous ridge of rather immobile material to a downstream screed or forming plate. To aid spreading of the slurry, the amount of water used in the slurry is often increased. However, the presence of large amounts of water is undesirable inasmuch as it must be removed before the final board product is obtained. Water removal is costly because of the energy required to evaporate the water and/or the time required to evaporate the water.
In addition, depending on the speed of the slurry being discharged, the cementitious slurry exiting the mixer can be turbulent such that undesirably large voids (e.g., voids having a diameter of 5 mm or greater) are formed as the slurry contacts the moving support surface. The presence of such large voids is undesirable during board production because it compromises the quality of the resulting board, including, for example, the strength of the board and the score and snap properties, and can lead to formation of blisters.
In the case of cement board, rollers and other devices have been used to spread out the slurry; however, such rollers and devices can suffer from slurry build up which requires them to be continuously cleaned during board production. Continuous cleaning creates a risk that a piece of built up set cement will fall off the rollers and other spreading devices and break the backing paper, or, in some applications, the release paper, thereby requiring complete shut down of the line. Interruption of a board production line is costly and inefficient.
Accordingly, it will be appreciated from the foregoing that there is a need in the art for a cementitious board system and production method that can reduce the amount of water, the number of undesirably large voids, and/or provide a more efficient, low cost board product. The invention provides a board system and methods that include one or more of such features. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.